Nd:YVO4, frequently referred to by practitioners simply as yttrium vanadate, is a preferred gain-medium for generating fundamental laser radiation at a wavelength of about 1064 nanometers (nm). The fundamental radiation can be used directly or frequency converted to provide second and higher harmonic wavelengths. The radiation can also be converted to non-harmonic wavelengths in an optical parametric oscillator arrangement (OPO) or the like. Yttrium vanadate is typically optically pumped by radiation from a diode-laser or an array of diode-lasers emitting radiation at a wavelength of about 808 nm which is a wavelength of peak absorption in Nd:YVO4. The optical pump-radiation is absorbed by the yttrium vanadate gain-medium, thereby energizing electrons of the gain-medium to an excited state. The excited-state electrons subsequently release energy acquired from the optical pumping as the fundamental laser radiation. Some proportion of the pump radiation energy absorbed by the gain-medium does not contribute to energizing electrons and heats the crystal.
A yttrium vanadate crystal is a tetragonal crystal having mutually perpendicular X, Y, and Z (alternatively, a, b, and c) crystalline axes. Yttrium vanadate crystals for use as optical-gain elements in lasers are usually cut from a much larger crystal boule of the material. The boule is usually grown by a Czochralski method with the boule growing along the Z-axis. Discs are sliced from the boule perpendicular to the Z-axis (growth direction) and the discs are cut into rectangular parallelepipeds for use as gain-elements. These gain elements (crystals) are cut such that they have a longitudinal axis perpendicular to the crystal Z-axis with either the X-Z planes or the Y-Z planes parallel to the longitudinal axis and are arranged such that when placed in a laser resonator, laser radiation propagates through the crystal in a X-Y plane perpendicular to the Z axis.
One preferred arrangement for optically pumping yttrium vanadate with diode-laser radiation is referred to by practitioners of the art as “end-pumping”. In end-pumping, optical pump radiation is delivered to the crystal generally along an axis aligned with the propagation axis (the resonator axis) of the laser radiation in the laser resonator. This method is preferred as it provides for efficient matching of the optical pump light with the mode volume of laser radiation propagating through the crystal. This optimizes efficiency of the laser and contributes to optimizing the quality of a beam of laser radiation delivered by the laser.
It has been found that in this end-pumping arrangement, yttrium vanadate crystals have a tendency to crack through the crystal or break completely during operation. This tendency becomes more evident as pump power into a given mode volume is increase. Cracking and breaking, however, can occur at a pump power less that what is contemplated as an optimum level or after a very short period of operation, if not immediately. This can create problems, inter alia, in providing an efficient laser, in limiting the maximum power that can reliably be extracted from a given laser design, or increasing material and manufacturing costs for a commercial supplier of yttrium vanadate based lasers.
It is believed that one explanation for the cracking tendency of the yttrium vanadate crystals is to be found in the manner in which pump light is absorbed in a crystal in a typical prior art pumping arrangement. One such prior art pumping arrangement is depicted in FIG. 1. Here, a laser 20 includes a laser resonator 22 terminated by mirrors 24 and 26. Resonator 22 includes an yttrium vanadate crystal 28. Resonator 22 has a longitudinal axis 30. An Nd:YVO4 crystal 28 is arranged in resonator 22 with the Z axis of the crystal aligned with resonator axis 30. Mirror 24 is coated for maximum reflection at the lasing wavelength and for maximum transmission at the pump light wavelength. Mirror 26 is partially transmitting at the lasing wavelength for coupling laser radiation out of resonator 22.
Pump light 32 for optically pumping crystal 28 is supplied by a diode laser array (not shown) via an optical fiber 34. Pump light 32 is unpolarized as a result of traveling through optical fiber 34. Typically the pump light has a wavelength of 808 nm. Nd:YVO4 has a strong absorption peak at this wavelength. The pump light is focused by a lens 36, through mirror 24, into yttrium vanadate crystal 28. Laser radiation generated as a result of the optical pumping circulates in resonator 22 along longitudinal axis 30 thereof as indicated by arrows F. The laser radiation is plane polarized in the Y-Z plane of the crystal as indicated by arrows P. Radiation F leaves resonator 22, via mirror 26, as output radiation.
Typically, pump light 32 is absorbed in the first one or two millimeters (mm) of the crystal and is concentrated in a circle of between about 800 and 900 micrometers (μm) about the Y-axis, or the X-axis depending on the crystal cut. This concentrated absorption causes a radial stress near the end of the crystal that can be sufficiently large that the crystal breaks (cleaves) along the X-Y plane. A more detailed description of his cleavage mechanism is provided in co-pending U.S. application Ser. No. 10/051,215, filed Jan. 18, 2002, assigned to the assignee of the present invention, the complete disclosure of which is hereby incorporated by reference. There is a need for an end pumping arrangement for yttrium vanadate crystals that can reduce the tendency of the crystals to cleave as a result of stresses imposed by strong optical pumping.